This invention relates in general to plant molecular biology and, more particularly, to a new class of glyphosate-tolerant 5-enolpyruvylshikimate-3-phosphate synthases.
Recent advances in genetic engineering have provided the requisite tools to transform plants to contain foreign genes. It is now possible to produce plants which have unique characteristics of agronomic importance. Certainly, one such advantageous trait is more cost effective, environmentally compatible weed control via herbicide tolerance. Herbicide-tolerant plants may reduce the need for tillage to control weeds thereby effectively reducing soil erosion.
One herbicide which is the subject of much investigation in this regard is N-phosphonomethylglycine commonly referred to as glyphosate. Glyphosate inhibits the shikimic acid pathway which leads to the biosynthesis of aromatic compounds including amino acids, plant hormones and vitamins. Specifically, glyphosate curbs the conversion of phosphoenolpyruvic acid (PEP) and 3-phosphoshikimic acid to 5-enolpyruvyl-3-phosphoshikimic acid by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (hereinafter referred to as EPSP synthase or EPSPS). For purposes of the present invention, the term “glyphosate” should be considered to include any herbicidally effective form of N-phosphonomethylglycine (including any salt thereof) and other forms which result in the production of the glyphosate anion in plants.
It has been shown that glyphosate-tolerant plants can be produced by inserting into the genome of the plant the capacity to produce a higher level of EPSP synthase in the chloroplast of the cell (Shah et al., 1986) which enzyme is preferably glyphosate-tolerant (Kishore et al. 1988). Variants of the wild-type EPSPS enzyme have been isolated which are glyphosate-tolerant as a result of alterations in the EPSPS amino acid coding sequence (Kishore and Shah, 1988; Schulz et al., 1984; Sost et al., 1984; Kishore et al., 1986). These variants typically have a higher Ki for glyphosate than the wild-type EPSPS enzyme which confers the glyphosate-tolerant phenotype, but these variants are also characterized by a high Km for PEP which makes the enzyme kinetically less efficient (Kishore and Shah, 1988; Sost et al., 1984; Schulz et al., 1984; Kishore et al., 1986; Sost and Amrhein, 1990). For example, the apparent Km for PEP and the apparent Ki for glyphosate for the native EPSPS from E. coli are 10 μM and 0.5 μM while for a glyphosate-tolerant isolate having a single amino acid substitution of an alanine for the glycine at position 96 these values are 220 μM and 4.0 mM, respectively. A number of glyphosate-tolerant plant variant EPSPS genes have been constructed by mutagenesis. Again, the glyphosate-tolerant EPSPS was impaired due to an increase in the Km for PEP and a slight reduction of the Vmax of the native plant enzyme (Kishore and Shah, 1988) thereby lowering the catalytic efficiency (Vmax/Km) of the enzyme. Since the kinetic constants of the variant enzymes are impaired with respect to PEP, it has been proposed that high levels of overproduction of the variant enzyme, 40-80 fold, would be required to maintain normal catalytic activity in plants in the presence of glyphosate (Kishore et al., 1988).
While such variant EPSP synthases have proved useful in obtaining transgenic plants tolerant to glyphospate, it would be increasingly beneficial to obtain an EPSP synthase that is highly glyphosate-tolerant while still kinetically efficient such that the amount of the glyphosate-tolerant EPSPS needed to be produced to maintain normal catalytic activity in the plant is reduced or that improved tolerance be obtained with the same expression level.
Previous studies have shown that EPSPS enzymes from different sources vary widely with respect to their degree of sensitivity to inhibition by glyphosate. A study of plant and bacterial EPSPS enzyme activity as a function of glyphosate concentration showed that there was a very wide range in the degree of sensitivity to glyphosate. The degree of sensitivity showed no correlation with any genus or species tested (Schulz et al., 1985). Insensitivity to glyphosate inhibition of the activity of the EPSPS from the Pseudomonas sp. PG2982 has also been reported but with no details of the studies (Fitzgibbon, 1988). In general, while such natural tolerance has been reported, there is no report suggesting the kinetic superiority of the naturally occurring bacterial phosphosate-tolerant EPSPS enzymes over those of mutated EPSPS enzymes nor have any of the genes been characterized. Similarly, there are no reports on the expression of naturally glyphosate-tolerant EPSPS enzymes in plants to confer glyphosate tolerance.
For purposes of the present invention the term “mature EPSP synthase” relates to the EPSPS polypeptide without the N-terminal chloroplast transit peptide. It is now known that the precursor form of the EPSP synthase in plants (with the transit peptide) is expressed and upon delivery to the chloroplast, the transit peptide is cleaved yielding the mature EPSP synthase. All numbering of amino acid positions are given with respect to the mature EPSP synthase (without chloroplast transit peptide leader) to facilitate comparison of EPSPS sequences from sources which have chloroplast transit peptides (i.e., plants and fungi) to sources which do not utilize a chloroplast targeting signal (i.e., bacteria).
In the amino acid sequences which follow, the standard single letter or three letter nomenclature are used. All peptide structures represented in the following description are shown in conventional format in which the amino group at the N-terminus appears to the left and the carboxyl group at the C-terminus at the right. Likewise, amino acid nomenclature for the naturally occurring amino acids found in protein is as follows: alanine (Ala;A), asparagine (Asn;N), aspartic acid (Asp;D), arginine (Arg;R), cysteine (Cys;C), glutamic acid (Glu;E), glutamine (Gln;Q), glycine (Gly;G), histidine (His;H), isoleucine (Ile;I), leucine (Leu;L), lysine (Lys;k), methionine (Met;M), phenylalanine (Phe;F), proline (Pro;P), serine (Ser;S), threonine (Thr;T), tryptophan (Trp;W), tyrosine (Tyr;Y), and valine (Val;V). An “X” is used when the amino acid residue is unknown and parentheses designate that an unambiguous assignment is not possible and the amino acid designation within the parentheses is the most probable estimate based on known information.
The term “nonpolar” amino acids include alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan, and methionine. The term “uncharged polar” amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine and glutamine. The term “charged polar” amino acids includes the “acidic” and “basic” amino acids. The term “acidic” amino acids includes aspartic acid and glutamic acid. The term “basic” amino acid includes lysine, arginine and histidine. The term “polar” amino acids includes both “charged polar” and “uncharged polar” amino acids.
Deoxyribonucleic acid (DNA) is a polymer comprising four mononucleotide units, dAMP (2′-Deoxyadenosine-5-monophosphate), dGMP (2′-Deoxyguanosine-5-monophosphate), dCMP (2′-Deoxycytosine-5-monophosphate) and dTMP (2′-Deoxythymosine-5-monophosphate) linked in various sequences by 3′,5′-phosphodiester bridges. The structural DNA consists of multiple nucleotide triplets called “codons” which code for the amino acids. The codons correspond to the various amino acids as follows: Arg (CGA, CGC, CGG, CGT, AGA, AGG); Leu (CTA, CTC, CTG, CTT, TTA, TTG); Ser (TCA, TCC, TCG, TCT, AGC, AGT); Thr (ACA, ACC, ACG, ACT); Pro (CCA, CCC, CCG, CCT); Ala (GCA, GCC, GCG, GCT); Gly (GGA, GGC, GGG, GGT); Ile (ATA, ATC, ATT); Val (GTA, GTC, GTG, GTT); Lys (AAA, AAG); Asn (AAC, AAT); Gla (CAA, CAG); His (CAC, CAT); Glu (GAA, GAG); Asp (GAC, GAT); Tyr (TAC, TAT); Cys (TGC, TGT); Phe (TTC, TTT); Met (ATG); and Trp (UGG). Moreover, due to the redundancy of the genetic code (i.e., more than one codon for all but two amino acids), there are many possible DNA sequences which may code for a particular amino acid sequence.